This invention relates to abradable seals, more particularly to the use of a laser to texture the surface of a seal material to provide improved abradability.
The efficiency of modern gas turbine engines depends upon a tight seal between the rotating components (blades) and the stationary component (shroud) in the fan, compressor and turbine. This seal is established by allowing the blades to cut (abrade) a groove in an abradable seal material which prevents a substantial volume of air from leaking past the blade tip. Traditionally the seal materials have been fabricated from woven metallic fibers or sintered metallic particles and brazed in place. While these materials are easily abraded due to their high internal porosity and low strength, their resistance to particle erosion is poor which results in rapid loss of material. This loss of material degrades the seal and the efficiency of the engine rapidly decreases. Seal materials in the more advanced engine utilize thermal sprayed coatings which perform the same function as the braided abradable seals, but which are easier to apply and easier to replace when an engine is overhauled. The properties of these sprayed seals can be varied to either maximize erosion resistance or maximize abradability. However, it has not been possible to maximize these two properties simultaneously.
The use of thermal spray powders to form abradable seals is known in the art as shown by U.S. Pat. No. 4,291,089. Such powders are used to form a coating on a substrate to provide an abradable seal, that is to say a coating which seals the space between the substrate and an adjacent surface movable relative thereto, and which is abraded to a controlled extent by relative movement between the substrate and the adjacent surface. Such a seal is initially formed by thermal spraying a powder onto the substrate to form a coating with a slightly greater thickness than the spacing between the substrate and the adjacent surface, so that the coating is abraded by relative movement between the substrate and the adjacent surface to a slightly lesser thickness corresponding to the spacing between the substrate and the adjacent surface so as to provide an efficient seal there between. Such seals are used for example on turbine or compressor blades of gas turbine engines, such as those used in aircraft, to provide a seal between the blades and the turbine or compressor housing.
One of the problems in providing a suitable abradable seal is to produce a thermally sprayed coating which, on the one hand has sufficient structural strength which nevertheless is low enough to provide abradability, and which, on the other hand, has a sufficiently high resistance to erosion by particles impinging on the abradable seal coating during use. For example, in the case of gas turbine or compressor blades, the seal coating is subjected to impingement by abrasive particles entrained in the air and ingested by the engine.
In one type of powder used to form abradable seal coatings, each powder particle has a central core of non-metallic solid material surrounded by a layer of metallic material, as described for example in U.S. Pat. No. 3,914,507. Such powders are known as composite powders, with the powder particles being known as composite powder particles. One composite powder of this kind which has been suggested has particles each having a core surrounded by nickel or nickel alloy, and abradable seal coatings formed by thermal spraying such powders have been useful as abradable seal coatings on compressors and turbines of aircraft gas turbine engines.